Wielding a mace properly is all about timing and momentum. Once the swing of the mace has begun, stopping it or slowing it down is difficult. The fighter is committed to not just the blow, but also the recoil. Begin your strike when the opponent is leaning forward, hopefully off balance. It is completely predictable that he will lean backward, so aim for a point behind his head. By the time the mace gets there, his head will be in its path.